This is building hypotheticals on top of hypotheticals. They can stop at Oklahoma and Texas if that were to happen, and if they don't get both, then they can add Kansas (or whomever, I know they are land grant and AAU but they were like 57th in the WSJ what's your sports team worth and the largest market in the state is 65th).
You're ignoring that they actually want Kansas. They really are a school that Big Ten presidents like and think would fit perfectly. True, they don't "have" to go to 18 if they get Oklahoma and Texas... but they actually want to. They like the scheduling 'symmetry' it creates, from everything I've heard. Plus, if the Big 12 is eaten alive, we will wind up with four super conferences anyhow and it would essentially create an 8-team playoff. At that point, they'll create a third subdivision or else the rest of FBS would be absorbed into FCS.